An inverter is used to provide AC current for a motor drive from a source of DC voltage such as an AC line rectified to energize a pair of DC busses with several hundred volts. Transistor switches selectively couple each phase of the motor to the high and low DC voltage busses for short periods to properly energize each phase of the motor. Pulse width modulation (PWM) is generally used to control each of the switches. When properly controlled this allows the frequency and magnitude of stator voltage applied to an induction motor so as to maintain constant flux in the motor over a wide speed range and to substantially reduce harmonics in the current supplied to the motor.
It is known to control the inverter switching digitally using a waveform generator in combination with a microcontroller programmed to instruct the waveform generator operation on the basis of desired motor operation and feedback information from the motor. The waveform generator then outputs inverter switching signals to energize the motor. The microcontroller affords an operator interface with the inverter. Application specific integrated circuits (ASIC) are known for electronic circuitry dedicated to a particular function and has several advantages. To effectively utilize such technology it is necessary to design the circuit in a manner to optimize its advantages and to avoid the inherent drawbacks. Such a design for waveform generation in an adjustable frequency drive requires the PWM algorithm which is used in the control be reduced to a set of gates and registers in a manner that minimizes the complexity of the circuit.